Polyether polyurethane foams stabilized with malic acid, citric acid, or nitrilotriacetic acid



United States Patent POLYETHER POLYURETHANE FOAMS STABIL- IZED WITHMALIC ACID, CITRIC ACID, 0R NITRILOTRIACETIC ACID Kenneth L. Brown,Arnionk, N.Y., assignor to Union Carbide Corporation, a corporation ofNew York No Drawing. Filed Apr. 26, 1960, Ser. No. 24,642

a 4 Claims. (Cl. 260-25) This invention relates to polyurethanecompositions, and more particularly to cellular foamed polyurethanes towhich improved stability and resistance to heat deterioration have beenimparted by the addition of certain polycarboxylic acids.

Synthetic urethane products derived from reactions involving isocyanateswith active hydrogen-containing compounds are rapidly becomingcompetitive with natural and synthetic rubbers. Urethane polymers formedby reactions of dior polyisocyanates with active hydrogencontainingcompounds, e.g., polyols, polyesters, polyesteramides, polyamides,water, polyalkylene ether glycols, etc., are readily foamed by internaldevelopment of carbon dioxide or by means of a blowing agent whichvaporizes at or below the temperature of the foaming mass. Morerecently, commercial interest has been directed to polyether-basedurethane foams prepared by the one-shot and prepolymer techniques inwhich catalysis is effected by an organic tin compound. Organic tincatalysts offer a considerable number of advantages among which arecontrollable reaction rates and effective use in small concentrations.One of the more important disadvantages common to such catalysts,however, is their tendency to promote deterioration of polyether-basedurethane foams at elevated temperatures. This is undesirable since thedeterioration results in a corresponding loss of desired physicalproperties, e.g., tensile strength, compression set, elongation andloadbearing properties, which thus limits the utility of the foam forits intended purpose.

The present invention is predicated on the discovery that polycarboxylicacids selected from the group of citric acid, malic acid andnitrilotriacetic acid are highly effective as stabilizers for cellularpolyurethane foams prepared from polyether-isocyanate reaction systemscatalyzed by an organic tin catalyst. It has been found that when aminor amount of the above-described polycarboxylic acids areincorporated in a polyether-isocyanate reaction system of the above typeand the mixture subsequently foamed by the one-shot, semiprepolymer orprepolymer technique, improved urethane foams are obtained whichelfectively resist deterioration at elevated temperatures and exhibit asignificant retention of desired physical properties. Although the exactmechanism of stabilization is not known, it appears that thedeterioration of polyether-based urethane foams at elevated temperaturesis due to an oxidative degradation which results from an attack onpolyether linkages by free radicals produced from the organic tincatalyst during the curing cycle of the foam. The polycarboxylic acidsare believed to prevent the cleavage or degradation of polyetherlinkages through their function as a chelating or sequestering agent forthe free radicals.

In carrying out the invention the polycarboxylic acids may be added tothe liquid polyether, the isocyanate, or the polyether-isocyanatereaction mixture. The mixture is then simultaneously or stepwise foamedin the presence of the organo-tin catalyst by internal development ofcarbon dioxide, or by means of a blowing agent which vaporizes at orbelow the temperature of the foaming mass.

The amount of polycarboxylic acid employed will vary,

with such considerations as concentration, type of tin 3,087,900Patented Apr. 30, 1963 ice catalyst used and polyether reactant. As ageneral guide, the amounts used range from about 0.001 to 5%, preferablyabout 0.01 to 1.0%, by weight, based on the total weight of thepolyether-isocyanate reaction mixture.

The polycarboxylic acids of the invention are effective stabilizers fora wide variety of polyurethane foams derived from the reaction ofpolyethers and isocyanates. The term polyether as used herein refers toa compound which has a molecular weight of at least about 250, aplurality of ether oxygens, and contains at least two active hydrogensas measured and determined by the Zerewitinoif method, J.A.C.S., vol.49, p. 3181 (1927).

Illustrative polyethers include polyoxyalkylene glycols such as thepolyoxyethylene glycols prepared by the addition of ethylene oxide towater, ethylene glycol or diethylene glycol; polyoxypropylene glycolsprepared by the addition of 1,2-propylene oxide to water, propyleneglycol or dipropylene glycol; mixed oxyethylene-oxypropylene polyglycolsprepared in a similar manner utilizing a mixture of ethylene oxide andpropylene oxide or a sequential addition of ethylene oxide and1,2-propylcne oxide; polyether glycols prepared by reacting ethyleneglycol, propylene oxide or mixtures thereof with monoand polynucleardihydroxy benzenes, e.g., catechol, resorcinol, hydroquinone, orcinol,2,2-bis(p-hydroxypheny1)propane, bis(p-hydroxyphenyl)methane, and thelike; polyethers prepared by reacting ethylene oxide, propylene oxide ormixtures thereof with aliphatic polyols such as glycerol, sorbitol,trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol, sucrose orglycosides, e.g., methyl, ethyl, propyl, butyl and Z-ethylhexylarabinoside, xyloside, fructoside, glucoside, rhamnoside, etc.;polyethers prepared by reacting ethylene oxide propylene oxide ormixtures thereof with alicyclic polyols such astetrarnethylolcyclohexanol; polyols containing a heterocyclic nucleussuch as 3,3,5-tris(hydroxymethyl)-5-methyl-4- hydroxytetrahydropyran and3,3,5,5-tetrakis(hydroxymethyl)-4-hydroxytetrahydropyran; or polyolscontaining an aromatic nucleus such as 2,2-bis(hydroxyphenyl) ethanol,pyrogallol, phloroglucinol, tris(hydroxyphenyl) alkanes, e.g.,l,1,3-tris(hydroxyphenyl)ethanes, and 1,1, 3-tris(hydroxyphenyDpropanes,etc., tetrakis(hydroxyphenyl)alkanes, e.g.,1,1,3,3-tetrakis(hydroxy-3-methylphenyl) propanes, 1, 1,5 ,5 -tetrakis(hydroxyphenyl) butanes, and the like.

Other suitable polyols include the ethylene oxide, propylene oxide andmixed oxide adducts of aliphatic polyamines such as ethylene diamine,triethylene diamine, etc.; aromatic polyamines such as 0-, m-, andp-phenylenediamine; 2,4- and 2,6-diaminotoluene; 2,6-diamino-pxylene;4,6-diamino-m-xylene; 2,4-diamino-m-xylene; 3,5- diamino-o-xylene;2,6-diaminopyridine; 1,4-naphthylenediarnine; benzidine; tolidine;4,4'-methylenedianiline; 4, 4",4"-m-ethylidynetrianiline, and the like.

The molecular weight of the polyether used should range from about 250to about 12,000 depending upon the characteristics desired in the foamedurethane product. As a general rule, cellular urethane foams of maximumrigidity are prepared by the use of polyethers having a molecular weightrange of about 250 to 1250; for semirigid foams the molecular weight ofthe polyether should be about 800 to 1800; and for flexible open-cellfoams R(NCG) wherein G is oxygen or sulfur, x is an integer of two ormore and R is an alkylene, substituted alkylene, arylene or substitutedarylene radical, a hydrocarbon, or substituted hydrocarbon containingone or more aryl --NCG bonds and one or more alkyl NCK bond. R can alsoinclude radicals such as RZR where Z may be a divalent moiety such as O,ORO, CO-, CO -S, S-R-S, SO etc. Examples of such compounds includehexamethylene diisocyanate, l,8diisocyanatop-mentane, xylylenediisocyanates,

( OCNCH CH CH OCH 2 1methyl-2,4-diisocyanatocyclohexane, phenylenediisocyanates, tolylene diisocyanates, ehlorophenylene diisocyanates,diphenylmethane 4,4 diisocyanate, naphthalene 1,5 diisocyanate,triphenylmethane-4,4,4"-triisocyanate, xylene-a,ot'-diisothiocyanate,and isopropylben- Zene-aA-diiSocyanate.

Further included are dimers and trimers of isocyanates and diisocyanatesand polymeric diisocyanates of the general formulae:

in which x and y are two or more, as well as compounds of the generalformula:

in which x is one or more and M is a monofunctional or polyfunctionalatom or group. Examples of this type include ethylphosphonicdiisocyanate, C H P(O) NCO) phenylphosphonic diisocyanate, C H P(NCO)compounds containing a ESlNCG group, isocyanates derived fromsulfonamides (RSO NCO), cyanic acid, thiocyanic acid, and compoundscontaining a metal-NCG group such as tributyltin isocyanate.

The preparation of polyether-based urethane foams can be carried out byforming a prepolymer, i.e., prereacting molar equivalents of thepolyether and isocyanate in the absence of water and thereafterproducing a foam by the addition of excess isocyanate, tin catalyst,water and surfactant; by the one-shot method in which the polyether,blowing agent, and isocyanate reactants are simultaneously mixedtogether and allowed to react in the presence of an organic tincatalyst; or by the semiprepolymer technique wherein the polyetherreactant is partially extended with excess isocyanate to provide areaction product containing a high percentage of free isocyanate groups(20-35%) which is then foamed at a later stage by reaction withadditional polyether, a blowing agent and tin catalyst.

The amount of isocyanate used in the preparation of flexible, rigid orsemirigid foams should be such that there is more than the theoreticalamount required to form a urethane linkage, NHCO-O, in the polymerresulting from reaction of the isocyanate with the active hydrogens ofthe polyether. The amount of isocyanate employed generaly ranges fromabout 1.0 to 7 equivalents, preferably 2 to 6 equivalents, perequivalent of polyether.

The reaction of excess diisocyanate with a polyoxypropylene glycolproduces a polymer having terminal isocyanate groups as illustrated bythe equation:

Excess OCN-RNGO HO(C2H O),1H

OCNR[NHO OO(CzHiO)nCzHi-OGONHRJNCO in which R represents an aliphatic,cycloaliphatic or aromatic diisocyanate exclusive of reactive isocyanategroups (-NCO), x is an integer greater than 1 and n is an integer suchthat the molecular weight of the ether glycol is at least 250. When itis desired to form a foam, the mixture of the isocyanate-modifiedpolyether reacts through the isocyanate groups with a chain extendingagent containing active hydrogen, e.g., water, in the presence of anorganic tin catalyst. This involves several reactions that proceedsimultaneously including the reaction 4 between the isocyanate groupsand water to form urylene links (NHCONH) and carbon dioxide, as well asthe reaction of the urylene links so formed with unreacted isocyanategroups to form biuret cross links. Depending upon the desired density ofthe urethane foam and the amount of cross linking desired, the total NCOequivalent to total active hydrogen equivalent should be such as toprovide a ratio of 0.8 to 1.2 equivalents of NCO per equivalent ofactive hydrogen and preferably a ratio of about 0.9 to 1.1 equivalents.

The foaming operation also can be eifected by means of a blowing agent,such as a low boiling, high molecular weight gas, which vaporizes at orbelow the temperature of the foaming mass. In rigid foams intended foruse in the field of insulation and structural reinforcement theincorporation of a gas lowers its heat conductivity. If a fluorocarbongas such as trichloromonofluoromethane, Ucon 11, is used in blowingrigid foams, a lower K- factor is obtained than in regid foams of equaldensity blown with air or carbon dioxide. The reactions that occurduring this type operation include formation of the urethane linkage aswell as the formation of isocyanates dimers and trimers. In addition,another reaction that can occur is the formation of allophanatestructures.

Preferred blowing agents are the fluorocarbons such astrichloromonofluoromethane; dichlorodifluoromethane,dichlorofiuoromethane, 1,l-dichloro-l-fluoroethane;l-chloro-l,l-difluoro, 2,2-dichloroethane; and 1,1,1-trifiuoro,2-chloro-2-fluoro, 3,3-difluoro, 4,4,4-trifluorobutane. The amount ofblowing agent used will vary with density desired in the foamed product.In general it may be stated that for grams of resin mix containing anaverage NCO/ OH ratio of 1 to 1, about 0.005 to 0.3 mole of gas are usedto provide densities ranging from 30 to 1 lbs. per cubic foot. Ifdesired, water may be used in conjunction with the blowing agent.

Organic tin catalysts that are suitable for accelerating thepolyether-isocyanate reaction are compounds having the general formula:

in which R represents hydrocarbon or substituted hydrocarbon radicalssuch as alkyl, aralkyl, aryl, alkaryl, alkoxy, cycloalkyl, alkenyl,cycloalkenyl, and analogous substituted hydrocarbon radicals; the Rrepresents hydrocarbon or substituted hydrocarbon radicals such as thosedesignated by the R or hydrogen or metal ions, the X representshydrogen, halogen, hydroxyl, amino, alkoxy, substituted alkoxy, acyloxy,substituted acyloxy, acyl radicals or organic residues connected to tinthrough a sulfide link; and the Y represents chalcogens including oxygenand sulfur.

'Among the compounds of group (a) that deserve special mention aretrimethyltin hydroxide, tributyltin hydroxide, trimethyltin chloride,trimethyltin bromide, tributyltin chloride, trioctyltin chloride,triphenyltin chloride, tributyltin hydride, triphenyltin hydride,triallyltin chloride, tributyltin fluoride, tributyltin acetate, andtetrabutyltin, etc.

The compounds in group (b) that deserve particular mention and arerepresentative of the group include dimethyltin diacetate, diethyltindiacetate, dibutyltin diacetate, dioctyltin diacetate, dilauryltindiacetate, dibutyltin dilaurate, dibutyltin maleate, dimethyltindichloride, dibutyltin dichloride, dioctyltin dichloride, diphenyltindichloride, diallyltin dibromide, diallyltin diiodide,bis(carboethoxymethyl)-tin diiodide, dibutyltin dimethoxide, dibutyltindibutoxide,

i a) 2 2( a z) x-1 2 3] 2 (in which x is a positive integer),dibutyl-bis[O-acetyl acetonyl]-tin, dibutyltin-bis(thiododecoxide), and

and [OH3O CH2 5] 28110 which the xs are positive integers).

Methylstannonic acid, ethylstannonic acid, butylstannonic acid,octylstannonic acid, HOOC(CH SnOO'I-I,

(CH N (CH SI1OOH cH OCH tcH Ocl-l CH SnOOH are representative of groupbutyltin and CH OCH (CH OCH CH O (CH SnO0H are examples of group (e)catalysts and group (j) catalysts are represented by HOOSn(CH ),,SnOOHand HOOSnCH (CH OOH CH SnOOH, the xs being positive integers.

Typical compounds in group (g) include compounds as poly(dialky1tinoxides) such as dibutyltin basic laurate and dibutyltin basic hexoxide.

Other compounds that are efficient catalysts are those of group (h), ofwhich the organo-tin compounds used as heat and light stabilizers forchlorinated polymers and available commercially for such use, aretypical.

Other organic tin compounds which can be used include the divalent tincompounds selected from the group consisting of stannous acylates andstannous alkoxides.

Suitable stannous acylates are the divalent tin salts of aliphaticmonoand polycarboxylic acids which contain from 1 to 54 carbon atoms.The acids can be saturated, such as acetic acid, 2-ethylhexanoic acid,etc.; unsaturated such as oleic acid, linoleic acid, ricinoleic acid,and the like; or they may be polymerized fatty acids derived fromnatural oils, e.g., linseed oil, tung oil, soybean oil, dehydratedcastor oil, which have a molecular weight up to about 500. Examples ofspecific acylates include: stannous acetate, stannous propionate,stannous oxalate, stannous tartrate, stannous butyrate, stannousvalerate, stannous caproate, stannous caprylate, stannous octoate,stannous laurate, stannous palmitate, stannous stearate, and stannousoleate. Of these materials the preferred catalysts are stannous acetate,stannous octoate and stannous oleate.

The stannous alkoxides which can be used may be represented by theformula:

in which R is a monovalent hydrocarbon radical, saturated orunsaturated, branched chain or straight chain, containing 1 to 18 carbonatoms, preferably 3 to 12. Representative examples of stannous alkoxidesinclude stannous methoxide, stannous isopropoxide, stannous butoxide,stannous t-butoxide, stannous Z-ethylhexoxide, stannous tridecanoxide,stannous heptadecanoxide, stan- .6 nous phenoxide, and 0-, mandpstannous cresoxides, etc.

Either class of stannous catalysts may be substituted with hydroxy, haloand keto, etc., groups.

The following examples illustrate the best mode now contemplated forcarrying out the invention.

EXAMPLE I A typical polyurethane foam formulation was prepared asfollows.

Material: [Parts by Weight Glycerol-propylene oxide adduct, 3,000

average molcular weight 100.0 Water (total) 2.9 Tetramethylbutanediamine 0.10 N-methyl morpholine 0.20 Dibutyltin dilaurate 0.20Emulsifier (siloxane-oxyalkylene copolymer) 0.80 Tolylenediisocyanate 38. l Additive 0.01 to 0.1

Polyurethane foams, 2" x 2" x 1", prepared from the above ingredientswere isolated in a quart can with vented lids and heated to C. The foamswere removed periodically and qualitatively examined for failure.Failure occurred when a pointed object, such as a pencil, would easilypenetrate the foam. Table 1 illustrates the effectiveness of thepolycarboxylic acids for imparting improved stability and resistance toheat deterioration.

What is claimed is:

1. In a method for preparing polyurethanes wherein a polyether polyolwhich has a molecular weight of at least about 250 and which contains atleast 2 active hydrogens as measured and determined by the Zerewitinofttest and an organic polyisocyanate are reacted together in the presenceof an organic tin catalyst and foamed by means of a blowing agent, theimprovement which comprises incorporating in the reaction mixture astabilizing amount of a polycarboxylic acid selected from the groupconsisting of malic acid, citric acid, and nitrilotriacetic acid.

2. The method of claim 1 wherein the acid is citric acid.

3. The method of claim 1 wherein the acid is malic acid.

4. The method of claim 1 triacetic acid.

wherein the acid is nitrilo- References Cited in the file of this patentUNITED STATES PATENTS 2,667,522 McElroy Jan. 26, 1954 2,894,919 Simon eta1. July 14, 1959 2,915,496 Swart et a1. Dec. 1, 1959 OTHER REFERENCESMobay: A One Shot System for Flexible Polyether- Urethane Foams,November 10, 1958.

Union Carbide, One-Step Urethane Foams, Advance Technical InformationSheet 1 -40487, February 9, 1959.

Union Carbide, Carbide Announces New Catalyst for Polyether UrethaneFoams, November 25, 1958.

1. IN A METHOD FOR PREPARING POLYURETHANES WHEREIN A POLYETHER POLYOLWHICH HAS A MOLECULAR WEIGHT OF AT LEAST ABOUT 250 AND WHICH CONTAINS ATLEAST 2 ACTIVE HYDROGENS AS MEASURED AND DETERMINED BY THE ZEREWITINOFFTEST AND AN ORGANIC POLYISOCYANATE ARE REACTED TOGETHER IN THE PRESENCEOF AN ORGANIC TIN CATALYST AND FOAMED BY MEANS OF A BLOWING AGENT, THEIMPROVEMENT WHICH COMPRISES INCORPORATING IN THE REACTION MIXTURE ASTABILIZING AMOUNT OF A POLYCARBOXYLIC ACID SELECTED FROM THE GROUPCONSITING OF MALIC ACID, CITRIC ACID, AND NITRILOTRIACETIC ACIDD.